Traditional voltage reference devices have included either Zener diodes or band-gap devices. The Zener diodes exhibit problems such as long-term drift, unpredictable temperature drift and poor manufacturability. Band-gap references suffer from voltage noise on the output signal due to the very high internal gain of the band-gap amplifier transistors. The band-gap voltage references provide low drift voltages by adding negative temperature drift voltages to positive temperature drift voltages in the appropriate proportion. As shown in the the prior art of FIG. 1, the negative coefficient voltage is the base-emitter voltage drop of an NPN transistor. The positive temperature coefficient voltage is the difference between the two base-emitter voltage drops amplified by twice the emitter ratio to R1 and R2. Therefore, the output voltage is: EQU Vbg=Vbe.sub.1 +2 (R1/R2)(Vbe.sub.2 -Vbe.sub.1) (1)
Since the resistors are typical low noise, a principal noise contribution to Vbg is from the transistor Vbe and, more specifically, from the difference between Vbe.sub.2 and Vbe.sub.1. The noise is more evident when it is realized that R1 is normally between five and six times R2 to get a zero voltage drift over changing temperature.